Image-converting device



March 1952 e. s. P. FREEMAN 2,587,830

IMAGE- CONVERTING DEVICE Filed June 15, 1950 A A Miami:010mm! o!10101010103010!10103101010 25 mulm GEORGE STANLEY PERCIVAL FREEMAN l4 INVENTOR.

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HIS ATTORNE Patented Mar. 4, 1952 IMAGE-CONVERTING DEVICE George Stanley Percival Freeman, Chiswick, London, England, assignor to Cinema-Television Limited, London, England, a British company Application June 15, 1950, Serial No. 168,231 In Great Britain June 29, 1949 9 Claims.

This invention relates to image-converting devices of the type, which may be utilized, for example, as television pick-up tubes or the like.

One of the diificulties encountered in television pick-up tubes of the image orthicon type is the loss of sensitivity caused by the mechanical blocking of photoelectrons originating in the image section of the tube by the capacitive grid or mesh of the storage electrode. Another disadvantage resides in the production of moir patterns produced in the transmitted image as a result of beating between the scanning beam and the capacitive grid or mesh, unless the electrode potentials are very critically adjusted. Furthermore, the reproduced signal may be distorted at high light levels because of incomplete collection of secondary electrons released by the primary photo-electrons at the target; this leads to impaired picture fidelity, as indicated by the gamma of the device, and in light or dark halos surrounding dark or bright areas respectively in the image. Moreover, the signal multiplication by secondary emission at the target is comparatively poor.

It is a primary object of the present invention to provide a new and improved image-converting device which avoids one or more of the disadvantages of prior art devices.

It is a further object of the invention to provide a novel target electrode which is particularly adapted for use in an image-converting device of the double-ended type.

A further object of the invention is to provide an image-converting device which is capable of efficient operation on two diflerent and distinct principles dependent upon the operating conditions, such as illumination and the like.

In accordance with the invention, a new and improved double-ended image-converting device comprises a storage electrode including a doublesided electron-impermeable target electrode, a thin imperforate layer of insulating material affixed to the target electrode, and a thin electronpermeable layer of conductive material affixed to the insulating layer. A pair of electron sources are respectively disposed on opposite sides of the storage electrode, and means are provided for projecting electrons from one of the sources through the conductive and insulating layers onto the target electrode to induce local transverse conduction currents in the insulating layer. Means are also provided for utilizing the conduction currents to efiect image conversion.

, In an image-converting device constructed in accordance with the invention, the charge pattern stored on the target electrode may be either positive or negative. Moreover, the gain of the image section of the device may be controlled without materially afiecting the focus of the photoelectrons incident on the storage electrode. As a still further feature, the device is adapted for use either with high-velocity scanning of the target as in the conventional image iconoscope, or with low-velocity scanning, as in the image orthicon. High-velocity scanning is advantageous under circumstances where high contrast and maximum picture resolution are desired, while low-velocity scanning permits a high light sensitivity to be attained.

The expression double-sided target electrode as used herein is intended to include any type of dou le-sided mosaic structure, as well as any type of thin uniform semi-conducting layer, of glass or the like, which exhibits the property of being transversely conductive while being substantially non-conductive in a lateral direction.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in connection with the accompanying drawing, in which:

Figures 1 and 2 are cross-sectional views, partly schematic, of image-converting devices constructed in accordance with the invention, and Figure 3 is a cross-sectional view of the storage electrode employed in the devices of Figures 1 and 2.

In the device of Figure l, a substantially plane photo-cathode I0 is supported adjacent the inner surface of the end wall of an evacuated envelope l l and an optical system l2, schematically represented by a single lens, is provided to focus an optical image on photo-cathode l0. Photoelectrons originating at cathode ID are directed to a storage electrode l3 which comprises a doublesided target electrode l4, preferably constructed of a thin layer of semi-conducting glass, as for example silica fused with a large percentage of lead oxide, coated on the side facing photo-cathode I0 with a very thin imperforate layer 15 of insulating material such as silica or magnesium fluoride. A thin layer It of conducting material. such as aluminium or beryllium, is ailixed to insulating layer IS. The conductive layer I6 is made sufiiciently thick to be electrically conductive but thin enough to permit easy penetration by electrons originating at photo-cathode H].

A second electron source, which may conveniently comprise a conventional scanning electron gun schematically represented at H, is provided within envelope H on the side of storage electrode i3,oppositephoto-cathode It, and suitable magnetic-deflection coils l8 and I9 are provided to impart the desired scanning motion to the electron beam originating at electron gun "l 7. An elongated focussing coil 20 surrounds the entire envelope II to collimate the electronsoriginating respectively at photo-cathodei l!- and electron gun l1. 7

The construction of electron gun ll andphotocathode I0 may assume any convenient form known to the art, and the storage electrode l3 may be supported in a conventional manner, as by means of a clamping ring (not shown) enga ing the inner wall of envelope I l. It known; in; the art that if an insulator ,is penetigated and traversed :by-a f ast; moving. stream eltr9ns; t srr ndersd qc l y: du vein a :tna sver :d tip ew th re wt g l ed over, .byethe application 0f suitable potentials ,to either surfaceof "the insulator, a conduction curvrent oiflan order *of magnitude larger than the penetrating current, may be caused to flow. -111.

.accordance with the present invention, the conduction current efiect is utilized as follows:

,.-Photoelectrons originating at cathode 10 are focussedonto the-storage electrode ls at high velocity and penetrate theconductive layer it and the insulating layer [5, coming to rest in the double-sided targetelectrodel 4. The target .elec- "trode is stabilized at. an equilibrium, potential by means. of .the scanning beam ;from electron. gun l7 which is deflected overthe target Min accordance with a predetermined. scannin 'pattern, .as by means of deflectioncoils I8.and-|9.. A suitable potential different from the equilibriumpotential of the target-electrode ,M, isapplied to the conductive ..layer= It .as vby means of a, lead-in ,con- .ductor .2I, which,;maycalso;.serve,,-as,an output terminal.

At theinstantoi passage of the primary photo electrons. .frorn cathode I through the insulating layer I5, a large conduction current, proportional in intensity to .the primary electron current, though perhaps ten times greater, flows from conductive layer 16, to target electrode It, thus locally altering the potential of target electrode Hi and establishing a space-modulated charge pattern,

corresponding to' the light-distribution from the opticalimage, projected on photo-cathode lG,-over the vscannedsurface of target electrode M. Since the storage electrode structure does not contain a grid-or mesh, the disadvantages of mcir patpositive or negative-withrespect to theequilibrium H potential which represents ---picture black. "-Small variations of--the-potential applied to conductive layer 16, which may,.for examplepbe effected by -means'of a'variabletap 22 coupled tov lead-in conductor 2i and associated with-apotentiometer .:..h i, 1 u at r(t ir tion-Ma s? abcam) 2 during the-passage 'o H the fast electrons. Moreing layers Ifiand l5 of, the storageelectrode l3. "Thus, thelightsensitivity ofthe tube, its contrast range, and its gamma characteristic may be brought under some degree of control to meet different conditions of operation.

-In onemode of operation, the scanning beam from electron gun ll-may comprise scanning electrons of low velocity, as in the image orthicon.

In'thismode of operation, conductive layer H3 is biased in such a manner that the stored charges :on target-electrode M are positive in sign. Since the target electrode [4 is electron-impermeable,

the scanning electrons are not collected by the conductive layer It, and a suitable output load impedance, such as a resistor 25, may be connected to layer l6 by means oflead-in conductor 2! which then servesas anoutput terminal, The .output signal so derivedis.more lfree; from noise than that of an .image orthicon wherein the ,re-

7 turn, electron beamis incident on a collector mesh. .If desired, the. return electron beam may be used to derive the output signal in a manner well known in the art, and electron, multiplication prior to the final output electrode may also be provided.

In another conditionof operation. the scanning beam from electron gun ll may comprise electrons of high velocity, as in the image iconoscope. Withthismode of operation the sign of the stored chargesmaybe made either positive or negative, and ineither case. conductive layer it mayconstitute theoutputelectrode. Moreover, if thestored chargesaremade positive with respect to pictureblack, the output signal may alternatively be derived from thereturn electron beam.

With either type of .operation,,the scanning beam from electron gun I! also serves to restore the target electrode M to itsequilibrium potential to condition the storage electrode l3 .for storage and conversion of a succeeding image frame.

Thus, the embodiment of Figure 1 is adapted to operation with either high-velocity or low-velocity scanning, depending on the operating conditions encountered. 'Moir patterns and mechanical blocking as Well as spurious shading signals are avoided.

In the device of Figure 1, the writing beam from photo-cathode l 9 is-caused to penetrate the insulating layer l 5 of the storage electrode l3, and the local transverse conduction currents induced in the thin insulating layer I5 are'utilized to establish acharge pattern on the semi-conducting target electrode H which is space-modulated in accordance with the optical image to be converted. It is also possible, in accordance with the invention, to cause the reading beam to penetrate the-thin insulating layer-and-th-us to utilize the local conduction currents for-thepurpose of establishing an output current flow to the thin conducting layer of the storage electrode,' thereby to produce an output signal :representing'the stored image. There is shown-in Figure 2 an image-converting device of thislatter type.

"The device of Figure 2,- with the exception-of the storage electrode, is generally similar to the conventional image iconoscope;=many parts of'the device are :similar to:corresponding parts "of the 7s tube of Figure '1 and; are --designated "by corresponding primed reference numerals. The storage electrode I3 is identical to that of the tube of Figure l but is reversely oriented with respect to the photo-cathode I and the scanning electron gun I'I; that is, the semi-conducting target electrode I4 is directly exposed to photoelectron emission from cathode I I3, while the reading electrons from scanning gun I? are caused to penetrate the conductive layer I6 and the insulating layer I of the storage electrode I3 in order to reach the target electrode I4. A focussing solenoid 26 is concentrically arranged with respect to the envelope I I around the image section of the tube to focus primary photoelectrons emitted from cathode I0, and a pair of anodes 21 and 28 are provided in the image section and the scanning section respectively for electrostatic focussing purposes; anodes 2'! and 28 may conveniently assume the form of conductive coatings on the inner wall of the envelope II. As in the embodiment of Figure 1, magnetic-deflection coils I8 and I9 are provided to impart the desired scanning motion to the electron beam originating at electron gun l1, and an output terminal 2| is connected to the conductive layer I6 of the storage electrode I3.

In operation, photoelectrons emitted from cathode III, which are space-modulated in accordance with the optical image to be converted, are focussed and impinge upon the semi-conducting target electrode I4 at a velocity, for example, of the order of 1000 volts, and secondary electrons are liberated from the target surface which may be suitably treated to provide a high secondary emission ratio. electrons from the target surface causes a positive charge to be transferred by leakage to the adjacent surface of the insulating layer I5, thereby to establish a charge pattern which is spaced-modulated to represent the optical image to be converted.

' A high-velocity scanning beam from electron gun I1 is directed over the surface of conducting layer I6 in accordance with a predetermined scanning pattern, and this reading beam penetrates both the conductive layer I6 and the insulating layer I5 to render the latter conductive in a transverse direction. As a result, the positive charge established on the insulator surface leaks through the insulating layer I5 to the conductive layer I6, by virtue of the local transverse conduction currents which are established in the insulator when it is traversed by an electron beam. A suitable potential diiferent from the equilibrium potential of the target electrode I 4 is applied to the conductive layer I6 by way of lead-in conductor 2i and at the instant of passage of the scanning electrons through an elementary area of the insulating layer 55', a large conduction current proportional in intensity to the primary current from the corresponding elementary area of the photo-cathode I0, though perhaps ten times greater, flows to the conductive layer It. By providing a suitable load impedance (not shown) in the circuit between conductive layer I6 and electron gun I'I, an electrical output signal which is modulated in accordance with the optical image to be transmitted may be produced, and lead-in conductor 2| may furnish a convenient output terminal from which the output signal may be derived.

The arrangement of Figure 2 possesses all the inherent advantages of the image iconoscope while avoiding its major disadvantages, namely, that the scanning electrons from the reading gun Emission of secondary 6. may penetrate the storage electrode and cause spurious signals by virtue of their subsequent interference with the operation of the image section of the tube. In the device of Figure 2, the semi-conducting target electrode l4 iselectronimpermeable so that the scanning electrons cannot penetrate the storage electrode and spurious signals are avoided. Moreover, the storage electrode is simple of construction and no substantial redistribution of electrons from the scanning beam is possible, so that shading signals are also avoided.

The construction of storage electrode l3 of the device of Figure 1 and storage electrode I3 of the device of Figure 2 is illustrated in cross-section in Figure 3.

It is also possible, in accordance with the invention, to utilize the storage electrode in certain other environments to meet difierent conditions of operation. For example, the storage electrode may be particularly useful in an imageconverting device of the image-storage or memory tube type. Such a device may advantageously be provided in the form of a uniaxial structure having reading and writing electron guns on opposite sides of a storage electrode which per se is constructed as illustrated in Figures 1 and 2. 'As in the other embodiments of the invention, local transverse conduction currents induced in the insulating layer are utilized to provide image storage and conversion.

In all of its applications, the storage electrode provided by the invention, comprising a target electrode to which is affixed a thin insulating layer, and a thin conductive layer afiixed to the insulating layer, affords numerous advantages over storage electrodes previously used in conventional image-converting devices. Having no mesh or grid, the storage electrode of the present invention places no limitation on obtainable picture resolution. Moreover, since the target electrode portion of the storage electrode is electron-impermeable, electrons from the scanning section of the tube are precluded from adversely affecting operation of the image section, and vice versa.

While particular embodiments of the present invention have been shown and described, it is apparent that various changes and modifications may be made, and it is therefore contemplated in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

' Iclaim:

l. A double-ended image-converting device comprising: a storage electrode including a double-sided electron-impermeable target electrode, a thin imperforate layer of insulating material affixed to said target electrode, and a thin electron-permeable layer of conductive material affixed to said insulating layer; a pair of electron sources respectively disposed on opposite sides of said storage electrode; means for projecting electrons from one of said sources through said layers onto said target electrode to induce local transverse conduction currents in said insulating layer; and means utilizing said conduction currents to efiect image conversion.

2. A double-ended image-converting device comprising: a storage electrode including a double-sided electron-impermeable target electrode, a thin imperforate layer of insulating material afhxed to said target electrode, and a thin electron-permeable layer of conductive material affixed to said insulating layer; a pair of electron sources respectively disposed on opposite sides of said storage electrode; means for projecting elec-- trons from one of said sourcesthroughsaid layers onto said target electrode to induce local transverse conduction currents in said insulating layer; and means, including means for projecting electrons from the other of said sources onto said target electrode, for utilizing said conduction currents to effect image conversion.

3. A double-ended image-converting device comprising: a storage electrode includingv a double-sided electron-impermeable target electrode, a thin imperforate layer of insulating material afiixed to said target electrode, and a thin electron-permeable layer of conductive material affixed to said insulating layer; a pair of electron sources respectively disposed on opposite sides of said storage electrode; means for projecting electrons from one of said sources through said layers onto said target electrode to induce local transverse conduction currents in said insulating layer; and means including an output terminal con nected to said conductive layer for application of an energizing potential thereto to utilizesaid conduction currents to effect image conversion.

4. A double-ended image-converting device comprising: a storage electrode including a double-sided electron-impermeable target electrode, a thin imperforate layer of insulating material afiixed to said target electrode, and a thin electron-permeable layer of conductive material affixed to said insulating layer; a pair of electron sources respectively disposed on opposite sides of said storage electrode; means for modulating electrons from a first one of said-sources in accordance with an optical image; means for pro jecting said modulated electrons from said first source through said layers onto said target electrode to induce local transverse conduction currents in said insulating layer thereby to establish a space-modulated charge pattern on said target electrode representing said optical image; and means, including means for projecting electrons from the other of said sources onto said target electrode, for converting said space-modulated charge pattern to an electrical signal.

5. A double-ended image-converting device comprising: a storage electrode including a double-sided electron-impermeable target electrode, a thin imperforate layer of insulating Iraterial afiixed to said target electrode, and a thin electron-permeable layer of conductive material aflixed to said insulating layer; a pair of electron sources respectively disposed on opposite sides of said storage electrode; means for modulating electrons from a first one of said sources in accordance with an optical image; means for projecting said modulated electrons from said first source onto Said target electrode to establish a Cit space-modulated charge pattern thereon repre- 60 sentingsaid optical image; means for projecting electrons from the other of said sources through said layers onto said target electrode to induce local transverse conduction currents in said insulating layer in accordance with said spacemodulated charge pattern; and means including an output terminal connected to said conductive layer for utilizing said conduction currents to effect conversion of said optical image to an electrical signal.

6. A double-ended image converting device comprising: a storage electrode including a double-sided electron-impermeable target electrode, a thin imperforate layer of insulating'material afiixed to said target electrode, anda thin electron-permeable layer of conductive material afiixed to said insulating layer; a pair of electron sources respectively disposed on opposite sidesof said storage electrode; means, including means for modulating electrons from a first one of said sources in accordance with an optical image, for storing a charge replica of said image on said storage electrode; means for'projecting electrons from one of said sources through said layers onto said target electrode to induce local transverse conduction currents in said insulating layer; means including an output terminal connected'to said conductive layer for application of an energizing potential thereto to utilize said conduction currents to effect image conversion; and means for varying said energizing potential to control the gain of said image-storing means.

7. A storage electrode for an image-converting device comprising: a double-sided semi-conducting glass electron-impermeable target electrode; a thin imperforate layer of insulating material afiixed to said target electrode; and a thin electron-permeable layer of conductive material amxed to said insulating layer.

8. A storage electrode for an image-converting device comprising: a double-sided electronimpermeable target electrode; a thin imperfor te layer of silica afl'ixed to said target electrode; and a thin electron-permeable layer of conductive material affixed to said silica layer.

9. A storage electrode for an image-converting device comprising: a double-sided electron-impermeable target electrode; a thin imperforate layer of magnesium fluoride affixed to said t rget electrode; and a thin electron-permeable layer of conductive material affixed to said first-mentioned layer.

GEORGE STANLEY PERCIVAL FREEMAN.

Name Date Heimann Mar. 24, 1942 Number 

